1. Field of the Invention
The present invention relates generally to communications systems and, more specifically, the present invention relates to wireless communications systems employing antenna arrays.
2. Background Information
Antenna arrays may be used in communications systems that transmit and/or receive radio frequency signals. Antenna arrays typically include a number of antennas that are spatially separated and may be employed in a number of different wireless applications including radio communications systems, cellular systems, television broadcasting, point to point systems, paging systems, medical applications or the like.
The use of antenna arrays in such systems often provide antenna performance improvements over the use of a single element antenna. These antenna performance improvements may include for received signals improved signal to noise ratio and interference rejection. Antenna performance improvements for transmitted signals may include improved directionality and hence less power towards other co-channel users, security and reduced transmit power requirements. Antenna arrays may be used for signal reception only, for signal transmission or for both signal reception and transmission.
A typical application of antenna array systems is in a wireless communication system. Examples include a cellular communication system and a wireless local loop system. Such wireless communications systems typically include one or more communications stations, generally called based stations, each communicating with its subscriber units, also called remote terminals and handsets. In cellular systems, the remote terminal is typically mobile, while in wireless local loop systems, the remote unit is typically in a fixed location.
The antenna array is typically at the base station, but may also be employed at a user terminal. Communication from the remote terminal to the base station is typically called the uplink and communication from the base station to the remote terminal is typically called the downlink. In time division duplex (TDD) systems, uplink and downlink communications with a particular remote terminal occur at the same frequency, but at different time slots. In frequency division duplex (FDD) systems, uplink and downlink communications with a particular remote terminal occur at different frequencies and may or may not occur at the same time.
Since different frequencies are used in the uplink and downlink communication channels in FDD, the behavior of the signals in the uplink and downlink communication channels between the base station and remote terminal are different. The differences are more apparent as the channels have more multipath components. In a multipath environment, scattering and/or reflections caused by, for example, buildings or the like result in amplitude and phase changes and/or multipath components in the communication signals. Multipath components behave differently for different frequencies and a communication signal arriving at an antenna array changes with the frequency. This makes downlink beamforming more challenging for FDD systems than TDD systems since the uplink and downlink frequencies are the same in TDD systems. Accordingly, the performance of a wireless communications system is impacted by the presence of scatterers in the environment.
Another factor that impacts wireless communication system performance is the presence of multiple remote terminals utilizing the same frequency or channel at the same time in the environment. As the number of separate sources or remote terminals utilizing the same frequency increases, the amount of interference in the communications channel also increases.